Which organism is the most common cause of pneumonia in elderly patients?

• A. Chlamydophila pneumoniae
• B. Klebsiella pneumoniae
• C. Mycoplasma pneumoniae
• D. Streptococcus pneumoniae

Answer: (D)

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia in adults, including the elderly. Its prevalence stems from its ability to colonize the upper airway and then invade the lung when host defenses are weakened by age and comorbidities. This pathogen typically produces a fast-onset, lobar pneumonia with productive sputum and fever, which is a common presentation in older patients. Atypical pathogens like Chlamydophila pneumoniae and Mycoplasma pneumoniae are more characteristic of younger adults and often cause nonproductive cough with interstitial-type infiltrates, not the classic lobar consolidation seen with pneumococcus. Klebsiella pneumoniae is important in people with alcohol use or aspiration risk and tends to cause more destructive, necrotizing pneumonia, sometimes with thick sputum, but it is not the most common cause in the elderly overall. So, the most frequent etiologic agent in elderly pneumonia remains Streptococcus pneumoniae.